The present invention has as object an improvement of the surface preparation of the steel parts, to be hot-dip galvanized and, more specifically, it refers to the application mode of the flux solution, for batch hot dip galvanizing processes, containing up to 0.1 wt % of aluminium. The choice of the chemical composition of the flux solution, together with its specific best mode, ensure an improved wetting of the fabricated steel parts during the immersion in the molten alloy and ensure an uniform and adherent coating to the substrate (cold and/or hot rolled steel).
It is known since long time that it is possible to improve certain performances, for example the oxidation resistance and more generally the corrosion resistance of fabricated parts, particularly with steel, by coating with metals such as Zinc, Cadmium, Aluminium or their alloy. Among the various type of coatings, particularly interesting are those based on the Zn—Al alloys, for their superior resistance in several aggressive environment, for their good mechanical characteristics and for their good surface appearance. Generally, metal coatings, may be obtained by immersion of parts into a molten metal bath or by electrolysis, in both: continuous or discontinuous processes.
Currently, batch processes are primarily dedicated to products of limited size, as for example screws, bolts, steelwork and the likes, even if they can be applied also for products of larger dimensions. However, the trend is for continuously coat parts of undefined size, such as strips, rods and wires, and then transform them in the final products, for example by cutting and cold drawing the strip.
However, these products have some drawbacks, for example have cut edges, without the protective coating, and so less resistant to the attack in aggressive environments; these drawbacks, because of the increasing demand of the market for high quality products, begin to overcome the benefit offered by the continuous coating processes. Therefore the interest for discontinuous coating processes, applied to fabricated parts, such as spars, brackets, and similar for cars, for shipbuilding, for appliances, etc., is increasing.
Obviously, there is also great interest for the discontinuous coating process of steel parts with Zinc-Aluminium alloys which, as mentioned above, have more high-temperature oxidation resistance and more corrosion resistance in several aggressive media.
However, so far it is very difficult to obtain good hot-dip coatings with Zinc-Aluminium alloys, as to make, also for batch processes, the surface preparation of the steel parts at high temperature in an hydrogen atmosphere, typical of continuous galvanizing, is expensive and impractical. Nonetheless the usual flux solution, based on an aqueous solution containing Zinc plus Ammonium chlorides, lose its effect, when the concentration of Aluminium, in the molten bath, exceeds 0.01 wt %
Steel parts, badly pre-treated, are not properly wetted by the molten alloy during hot-dip, and the final coating will have black-spots and un-coated areas. It is worthwhile to recall briefly here, the scope of the fluxing pre-treatment before hot-dip galvanizing. The pre-treatment should remove all residual oxidation from the surface of the steel parts, also after acid pickling and would protect the surface, during immersion into the molten bath. The flux reacts with the Zn-alloy at 450° C. producing reducing gaseous components which protect against oxidation and are readily removed.
However, Al, already at very low percentages in the Zn-based alloy-bath, reacts as mentioned above, producing stable compounds, mainly oxides, which sticks on the surface and do not allow good wettability of the steel parts by the molten alloy, producing extended surface defects.
Many attempts have been made for the set-up of a robust batch process for Zn—Al-alloy coatings.
The U.S. Pat. No. 6,270,842 proposes a new flux composition, including NaCl and/or alkaline metals and NaF, to be used in batch coating processes for steel parts with Zn—Al.
The U.S. Pat. No. 6,221,431 proposes a new flux composition containing a mixture of salts of the cations: Ni, Al, K, and Mn for coating fabricated parts with so-called reactive steels.
A non conventional route is instead put forward by the U.S. Pat. Nos. 6,200,636 and 6,372,296, which refer to the chemical deposition of thin layer of metals, 5 to 50 nm thick, plated electroless, on a steel part, before hot-dip galvanizing into Zn-based or Zn—Al-alloys. The selected metals are: Sn, Cu, Ni, Co, Mn, Zr, Cr, Pb, Hg, Au, Ag, Pt, Pd, Mo, alone or in combination to each other.
The molten bath is either pure Zinc or a Zn—Al-alloy, containing Al up to 40%.
In the Japanese patent JP 05-117835, BiCl3 or SnCl2 or an alcohol, are added to the flux solution containing ZnCl2-NH4Cl, for Zn—Al coatings with Al between 0.001 and 20 wt %. It is also stated that it is not possible to flux wet steel parts and it is proposed a method for rapid drying the fabricated part after flux through controlled additions of a volatile aliphatic alcohol.
The U.S. Pat. No. 6,248,122 is relevant to the deposition of a continuous thin metallic film, followed by the immersion of the part into HCl before the hot-dip immersion into a Zn—Al molten alloy; the thus formed chloride would melt and facilitate the metal film to dissolve into the molten bath. The metallic film would protect the steel part surface against oxidation, which may cause defects on the final ZnAl coatings.
In the U.S. Pat. No. 6,921,543 the suggested composition of the flux is: 60-80 wt % ZnCl2, 7-20 wt % NH4Cl, 7-20 wt % of at least one alkaline or alkaline-earth salt, 0.1-0.5 wt % of a compound selected among NiCl2, CoCl2, MnCl2 and 0.1-1.5 wt % of at least one compound selected among PbCl2, SnCl2, BiCl3, SbCl3. Furthermore it is stated that the percentage of ZnCl2 is ranging between 70 and 78 wt % and that of NH4Cl between 11 and 15 wt %. The total salts dissolved into water is in the range 200-700 g/L, preferably 500-550 g/L. The molten Zn-bath contains Al between 0 and 56 wt %.
In the text it is clearly stated that: 1) the flux after drying is deposited on the surface of the parts; 2) the suggested quantities of ZnCl2 form a continuous film, on the surface to be galvanized; 3) the NH4Cl attacks the surface of the parts eliminating the residual rust or similar; 4) the chlorides of the alkaline, alkaline-earth, Lead, Tin, Bismuth, and Antimony metals, improve wettability of the part when immersed into the molten alloy. It is worth noting that in the examples in the text, the quantity of Al in the molten bath is not less than 4.2 wt %. This US patent corresponds to the EP 1 352 1000.
The EP 1 466 029, is relevant to the surface preparation before hot-dip galvanizing of steel parts cleaned in order to achieve a pollution level inferior to 0.6 μg/cm2; the cleaning treatment is followed by the immersion of the parts into a flux solution containing a soluble Bi salt which forms a protective layer. When the galvanizing bath is “galfan”, in order to achieve good results, the flux must guarantee the formation of a thin protective metallic layer on the steel part. In Claim 6 the flux must be an aqueous solution containing 0.3-2 wt % of Bi (as soluble salt, oxide, chloride, etc.). In Claim 11 the molten Zn-based bath must contain at least 0.15 wt % Al.
In the Italian Patent RM02A0589 the aqueous flux solution must contain 5-300 g/L of NH4Cl, 90-100 g/L of ZnCl2, 1-20 g/L of Bi chlorides, preferably in the following order: 10-150, 100-200, 1-10 g/L. This flux solution is able to plate a metallic layer (Bismuth), on the surface, whose thickness is between 1 nm and 1 μm. The flux solution may contain H3BiO4 and the galvanizing bath may contain 0.001-0.1 wt % Al.
The Italian patent RM05A0006 restricts the composition range of the flux: 10-1050 g/L NH4Cl; 80-270 g/L ZnCl2; 0.5-10 g/L BiCl3; 1-10 g/L CuCl2. The pH of the solution should be 1.8-2.3 corrected with HCl or NaCl. To the flux solution may be added: KCl (2-50 g/L, preferably 3-6 g/L) and/or SnCl2 (2-7 g/L, preferably 4-6 g/L or more preferable 3-5 g/L). The flux solution may also contain Bismuth oxide (1-16 g/L).
At last the international patent application WO 07/071039 (equivalent to the U.S. patent application Ser. No. 07/0,137,731) in which the aqueous flux solution contains 15-40 wt % ZnCl2; 1-10 wt % NH4Cl; 1-6 wt % of an alkaline-metal chloride; 0.02-0.15 wt % of a non-ionic surfactant, containing polioxy-ethylene alcohols, with a ratio between hydrophilic/lyophilic<11, brought to pH≦1.5 with the addition of an acid. The flux contains FeCl3 (1-4 wt %) and/or 0.05 wt % Bi2O3.
None of the techniques described above are satisfactory, as far as applicability, safety or environment. As an example, the use of an alcohol in a hot-dip galvanizing shop, where some of the process operations occur at high temperature, is not appropriate, for the fire danger and the gaseous emissions. Furthermore, the use of fluorides is not acceptable, being them dangerous for the environment and because of the high cost of exhaust disposal.
The innovation, based on the chemical deposition of a thin metallic film on the surface of the steel parts, followed by conversion in HCl, is costly as it introduces into the process an additional stage and it is not robust enough, as it depends on the reaction with HCl which is affected by residual surface pollution.
Furthermore, in the most recent Patent literature, the immersion time and the temperature of the flux solution are not mentioned, nevertheless it has been found they are very important when associated with the pH and to the flux concentration.
In any case, the batch coating process with Zn—Al-alloys present always many difficulties, caused primarily to surface cleanliness which should be maintained clean until immersion into the molten bath; this leads to coating defects, such as rough surfaces, poor adherence, black-spots, etc.